The present invention relates generally to hot dip coated galvanized steel strip and more particularly to a differentially coated, galvanized steel strip wherein one of two opposite strip sides has a relatively light coat composed of iron-zinc alloy and the other strip side has a relatively heavy coat at least the outer part of which consists essentially of zinc. The present invention also relates particularly to methods and apparatuses for producing such a differentially coated, galvanized steel strip.
A differentially coated, galvanized steel strip of the general type described in the preceding paragraph, and a method and apparatus for producing such a galvanized steel strip is described in Patil et al., U.S. Pat. No. 4,171,394 issued Oct. 16, 1979, and entitled "Process of Hot-dip Galvanizing and Alloying". The disclosure of said Patil et al. patent is incorporated herein by reference.
A differentially coated, galvanized steel strip of the general type described in said Patil et al. patent is produced by passing a continuous steel strip through a bath of molten coating metal consisting essentially of zinc to coat both sides of the strip with the molten coating metal. Immediately upon withdrawal of the coated metal strip from the bath of molten coating metal, the weight of the molten coating metal on opposite sides of the strip is adjusted by impingement against opposite sides of the strip of jets of gas or steam. The jets on respective opposite sides of the strip are adjusted to provide one strip side with a relatively light coat of the coating metal and the other strip side with a relatively heavy coat of the coating metal. Typically, the weight of the coating metal on the light coated side is in the range 0.05-0.25 oz./ft..sup.2 (0.015-0.075 kg/m.sup.2), and the weight of the coating metal on the heavy coated side is in the range 0.35-1.0 oz./ft..sup.2 (0.105-0.30 kg/m.sup.2).
Immediately following the weight adjusting step, the strip is passed through a treating zone wherein simultaneously the light coated strip side undergoes heating and the heavy coated strip side undergoes cooling. As a result, the coating metal on the light coated side is transformed to iron-zinc alloy throughout. It is desired, as a result of the treating step, that the coating metal on the heavy coated side be only partially transformed to iron-zinc alloy, and that the outer part of the coating on the heavy coated side consists essentially of zinc. In a method in accordance with the Patil et al. patent, the heavy coated side is at least partially molten at the time the simultaneous heating and cooling step is performed.
In a typical commercial product employing the subject matter of the Patil et al. patent, the weight of the coating metal on the heavy coated side is about 0.50 oz./ft..sup.2 (0.15 kg./m.sup.2). In such a product, the iron-zinc alloy on the heavy coated side constitutes an inner layer which ranges from 30 to 50% of the coating thickness on that side. Occasionally, however, as a result of the simultaneous heating and cooling step, in some spots the iron-zinc alloy may extend all the way through to the outer surface on the heavy coated side causing a defect known as "bleed-through". These bleed-throughs occur intermittently along the outer surface of the heavy coated strip side.
In cross-section, a bleed-through resembles the vertical cross-section of a mushroom. Bleed-throughs are undesirable because the iron-zinc alloy in the bleed-through area at the strip's outer surface tends to powder when the galvanized steel strip is subjected to a stamping operation, and this is undesirable. Although the light coated side of the strip consists entirely of iron-zinc alloy throughout, that coating is relatively so thin that it can undergo a stamping operation without powdering. The fully alloyed light coated side is readily paintable.
Galvanized steel strip with bleed-through on the heavy coated side is not acceptable to those who fabricate the galvanized steel strip into products, and such a strip is not saleable. A typical customer for differentially coated galvanized steel strip is a stamping shop making parts for the automotive industry.
Recently there has been a demand, particularly from the automotive industry, for a differentially coated, galvanized steel strip in which the heavy coated side has a thinner coating, e.g., a weight substantially less than 0.50 oz./ft..sup.2 (0.15 kg/m.sup.2), typically in the range 0.25-0.45 oz./ft..sup.2 (0.075-0.135 kg/m.sup.2). A thinner coating on the heavy coated side makes the strip more weldable. However, the thinner the coating on the heavy coated side, the more likely there is to be a bleed-through when the differentially coated strip is subjected to a simultaneous heating and cooling treatment of the type described in the Patil et al. patent.
When the heavy coated side has a coating weight of 0.50 oz./ft..sup.2 (0.15 kg/m.sup.2), a strip with a bleed-through occurs only occasionally, and the strip rejection rate for this defect is about 3-4%. When the coating on the heavy coated side has a weight substantially less than 0.50 oz./ft..sup.2 (0.15 kg/m.sup.2), it is virtually impossible to prevent bleed-throughs when employing a process in accordance with the Patil et al. patent, and the iron-zinc alloy may constitute 100% of the coating thickness on the heavy coated side.